Fluxes play an important role in solder-joining electronic components, such as semiconductor devices, onto printed circuit cards or printed circuit boards (PCBs). Flux is used in a process of flip-chip joining to a substrate that has ball grid arrays (BGA) or land grid arrays (LGA). In a typical process, by way of example, flux is applied onto a substrate followed by placing a chip onto the flux-applied substrate. Then, the chip-substrate module goes through a reflow process at a high temperature so as to make solder connections. The subsequently formed flux residue is cleaned (for example, with water) followed by drying the module. An underfill material is introduced into the gap between the chip and the substrate to maintain the integrity of the flip-chip package.
As the density of Controlled Collapse Chip Connection (C4) arrays and the chip size increase, the joining process sometimes produces non-wets at a corner of the large chip due to the smaller C4 size and the larger warpage of a substrate. It also becomes more difficult to clean flux residue, formed during flip-chip joining, out of the narrow chip-substrate gap of the large chip package.
Non-wets which make electrical open should be avoided, and flux residue often causes delamination between underfill and chip or between underfill and substrate, resulting in failure of flip-chip packages. As the size of solder balls in a chip decrease, slight movement of an aligned chip-laminate module during a reflow process can cause non-wets, because a typical high-throughput reflow tool tends to vibrate. Such non-wets can increase in the case of multi-chip modules.
Chemical components of a flux or its impurities can decompose or vaporize during reflow to give out vapors, which can cause chips to move out of position. Highly tacky fluxes can, however, prevent movement of chips even though vapors are coming out or a reflow furnace vibrates. Also, chemical components of the flux should have a boiling point that is higher than a typical reflow temperature in order to avoid movement of chips due to boiling. Solvent molecules of the flux, if they do not evaporate upon reflow due to their high boiling points, should be large enough so as not to diffuse into a substrate outer layer such as solder mask, which is relatively porous because it is a photoresist with silica fillers.
However, after joining, left-over flux and by-products of the reaction between flux and solders need to be cleaned, preferably with a low cost and environment friendly method because cleaning with organic solvent is not only harmful to human beings and the environment but also expensive, in regards to both the material itself and the waste treatment process. Therefore, it would be desirable to develop a flux that has sufficient tackiness and a sufficiently high boiling point, while the flux residue can be cleaned with water.
Many existing fluxes in the industry, however, provide an inadequate joining capability in case of lead-free solders, give non-wets at the corner of a chip, leave considerable flux residue onto chip, C4 and substrate surfaces after cleaning, diffuse into the substrate outer layer, and/or cause chip movement during reflow.
By way of example, existing fluxes include disadvantageous aspects such as being small enough to diffuse into a solder mask layer at a reflow temperature, resulting in a failure of the semiconductor package, not providing enough tackiness or viscosity, as well as often leaving residue after reflow that cannot be cleaned with water.
Accordingly, there is a need for soldering flux compositions that can effectively manufacture the modern and high-performance semiconductor packages used for computers, communication devices, home electronics, game consoles, audio/video equipments, automobiles, etc.